This invention relates to polymeric alloys composed of rod-like, aromatic, heterocyclic polymers and coil-like, aromatic, heterocyclic polymers. In one aspect it relates to a method for the preparation of composite films at the molecular level that are analogous to chopped fiber composites. In another aspect, it relates to composite films prepared from para ordered, rod-like, aromatic, heterocyclic polymers embedded in an amorphous, heterocyclic system.
Recent developments in the synthesis of rod-like, aromatic, heterocyclic polymers and the molecular composite films produced thereby have generated a continued interest in the development of these polymers as structural materials. As a consequence, a continuing research effort has been undertaken to improve their characteristics and to develop them into more useful product forms, such as fibers, films, or sheets. Basically, the research effort is directed toward the preparation and processing of very high strength, environmentally resistant, polymers for use as structural materials in aerospace vehicles. The objective is the attainment of mechanical properties for a structural material comparable with those currently obtained with fiber reinforced composites, but with significantly higher environmental resistance and without the use of a fiber reinforcement.
The materials found to be most successful for achieving the desired objective of this effort are rigid-rod, extended-chain, aromatic, heterocyclic polymers of the type disclosed in U.S. Pat. Nos. 4,108,835 and 4,207,407. However, these materials presented special processing problems because of the extended-chain, rigid-rod, structural character of their molecules. Previous processing techniques required strong mineral or organic acid solvents and there was little opportunity to influence the polymer morphology once the material was in the solid state.
In accordance with the invention defined in U.S. Pat. No. 4,207,407, however, it was found that this problem could be overcome by a concept which consisted of blending a rod-like, aromatic, heterocyclic polymer with a coil-like, aromatic, heterocyclic polymer. The intent is to reinforce the coil-like or amorphous polymer with the rod-like polymer, thus forming a composite on the molecular level analogous to chopped fiber reinforced composites.
In accordance with that invention, various polymer blends were processed as thin films by vacuum casting from dilute solutions. In general, the procedure followed was to prepare a 1 to 2 percent by weight polymer solution of the polymer blend in methane sulfonic acid and put the solution in a specially fabricated, circular, flat bottomed, casting dish. The dish was then placed and leveled in the bottom of a sublimator. The cold finger of the sublimator was maintained at -25.degree. C. and the sublimator was continuously evacuated and heated to 60.degree. C. to facilitate the removal of the methane sulfonic acid. After the films were formed and removed from the casting dish, they were generally dried at 100.degree. C. in a vacuum oven for 24 to 48 hours. The films produced were approximately 5-cm in diameter and varied from 1.3.times.10.sup.-3 to 16.5.times.10.sup.-3 cm in thickness. Most of the films retained approximately 20 to 30 percent residual solvent.
The high potential utilization of these rod-like polymers as molecular composite films in the fabrication of structural elements for aerospace applications provided the impetus for the continuing research effort referred to above and the attempt to provide even greater improvement in the characteristics of the film composites.
It was found, as a result of this effort, that greater and more efficient dispersion of the rod-like polymer in the amorphous polymer matrix could be achieved by a processing technique involving precipitation instead of the vacuum casting, film forming technique disclosed in U.S. Pat. No. 4,207,407. As a result, the mechanical properties of the molecular composite films of this invention are considerably improved when compared to those exhibited by the vacuum cast films previously fabricated, thus indicating excellent translation of rod properties.